Allow a NULL output string.

Author: katef

include/re/groups.h

@@ -37,6 +37,9 @@ struct re_pos;
  * The output string will always be less than or equal in
  * length to the format string. The output is \0-terminated.
  * outn includes the \0.
+ *
+ * outs may be NULL in which case outn must be 0, and no
+ * output is made.
  */
 bool
 re_interpolate_groups(const char *fmt, char esc,

src/libre/re_interpolate_groups.c

@@ -15,14 +15,14 @@
 #include <re/groups.h>
 
 #define OUT_CHAR(c)                  \
-    do {                             \
+    do if (outs != NULL) {           \
         if (outn < 1) goto overflow; \
         *outs++ = (c);               \
         outn--;                      \
     } while (0)
 
 #define OUT_GROUP(s)                 \
-    do {                             \
+    do if (outs != NULL) {           \
         size_t n = strlen((s));      \
         if (outn < n) goto overflow; \
         (void) memcpy(outs, s, n);   \
@@ -48,7 +48,7 @@ re_interpolate_groups(const char *fmt, char esc,
 	assert(esc != '\0');
 	assert(group0 != NULL || groupc == 0);
 	assert(groupc < UINT_MAX / 10 - 1);
-	assert(outs != NULL);
+	assert(outs != NULL || outn == 0);
 
 	state = STATE_LIT;
 	group = 0;

tests/re_interpolate_groups/re_interpolate_groups2.c

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2026 Katherine Flavel
+ *
+ * See LICENCE for the full copyright terms.
+ */
+
+#include <assert.h>
+#include <stdbool.h>
+#include <string.h>
+#include <stdio.h>
+
+#include <re/re.h>
+#include <re/groups.h>
+
+static unsigned failed;
+
+static void
+test(const char *fmt, bool expected)
+{
+	bool r;
+
+	assert(fmt != NULL);
+
+	r = re_interpolate_groups(fmt, '$', "<g0>", 0, NULL, "<ne>", NULL, 0, NULL, NULL);
+
+	failed += r != expected;
+
+	printf("%s/%d => %d%s\n", fmt, 0, r,
+		r != expected ? " XXX" : "");
+}
+
+int main(void) {
+	test("", true);
+	test("abc", true);
+	test("$$", true);
+	test("$x", false);
+
+	return failed;
+}
+